Leaf rust, caused by the fungus Puccinia triticina tritici and stripe (yellow) rust, caused by Puccinia striiformis tritici, are major wheat diseases. Leaf rust and stripe rust cause tremendous yield losses annually. In the last years stripe rust outbreaks were reported in Australia, China, Pakistan, Central and West Asia, the Middle East (Syria and Turkey), India and U.S.A., indicating virulence changes of the pathogen (Wellings C R et al., 2012. CAB International, pp. 63-83). It was also shown that new stripe rust strains became adapted to higher inoculation temperatures that may account for the hazardous spread of the pathogen (Milus E A et al., 2006. Plant Disease, 90:847-852)
Sharon goatgrass (Aegilops sharonensis Eig) (AES) is a wild diploid (genome SshSsh; 2n=14) relative of wheat. It is native to the coastal plain of Israel and south Lebanon, growing on stabilized dunes. Work done by Olivera et al. (Olivera P. D. et al. 2007. Plant Disease 91:942-950) on a representative sample of Sharon goatgrass lines collected in Israel and data from the Institute for Cereal Crops Improvement (ICCI, Israel) (Anikster Y. et al. 2005. Plant Disease 89:303-308) revealed that many accessions are highly resistant to inoculation with leaf rust or stripe rust pathogens. A recent evaluation of 1800 newly collected AES accessions at the ICCI confirmed the high frequency of resistance to these diseases in the species. Genetic analysis in a number of these lines (Olivera P. D. et al. 2008. Phytopathology 98:353-358) demonstrated monogenic inheritance of the resistance genes.
Although the Sharon goatgrass Ssh genome is closely related to the B genome of tetraploid and hexaploid wheat, the two genomes cannot be regarded as being homologous. Gene transfer from Sharon goatgrass may therefore be more difficult as compared to transfer from donor species with homologous genomes. Technical problems (e.g. timing of flowering, time of anther dehiscence) and inherent low crossability with wheat result in very low hybrid seed set. Thereafter, pairing and chromosome segment exchange is rare.
Different procedures have been utilized to transfer genes from wild relatives to wheat (e.g. Feldman M. 1983. Acta Biol. Yugoslay. Genet. 15: 145-161; Millet E. 2007. Isr. J. Plant Sci. 55:277-287; Millet E et al., 2007. CAB International pp. 554-563; Qi L et al., 2007. Chrom. Res. 15:3-19; Kilian B et al., 2011. Aegilops. In Wild Crop Relatives: Genomic and Breeding Resources, Cereals. Edited by C. Kole Springer-Verlag, Berlin Heidelberg, pp. 1-76), many of which included production of an amphiploid by chromosome duplication of the interspecific hybrid and use of mutants of the Ph genes, which suppresses homoeologous pairing, and particularly the ph1b allele to allow such pairing.
In addition, Sharon goatgrass possesses gametocidal (Gc) genes (Maan S S. 1975. Crop Sci. 15:287-292; Endo T R. 1985. Jpn. J. Genet. 60: 125-135). Only few AES accessions have been used in genetic studies, but all of them showed a gametocidal effect as reflected in the failure to obtain the whole pure series of addition lines of AES. The finding that chromosome 4Ssh was always included in breeding progenies (Zhang H. et al 2001. Theor. Appl. Genet 103:518-525), supports the contention that Gc genes cause preferential transmission of their hosting chromosome. Their presence in a plant is accompanied by chromosome breakage of gametes not carrying the Gc genes, ultimately leading to semi-sterile spikes.
To avoid this gametocidal effect, an “anti-gametocidal” wheat mutant (Gc2mut; Friebe B. et al. 2003. Chromosoma 111:509-517) that confers normal chromosome segregation rather than preferential transmission of the chromosome carrying the gametocidal gene may be used.
Despite its high resistance to different wheat diseases, Sharon goatgrass has hardly been exploited to improve wheat. Marais et al. (Marais G F et al., 2003. S Afr J Plant Soil 20:193-198) have identified potential useful resistance genes in Sharon goatgrass that were introgressed into common wheat chromosomes. In a further work leaf rust and stripe rust resistance genes, designated Lr56/Yr38, were transferred from Sharon goatgrass to chromosome 6A of common wheat (Marais G F et al. 2006. Euphytica 149:373-380). The translocation break occurred in the area of the long arm of wheat chromosome 6A. The Lr56/Yr38 translocation chromosome was found in effect to be most of the Sharon goatgrass chromosome with the terminal segment of its long arm replaced by a corresponding segment of wheat 6AL chromosome. In an attempt to reduce the amount of the transferred chromatin they employed recombination in the absence of the homoeologous pairing suppressor gene, Ph1, and obtained an intercalary sub-telomeric small introgression carrying the Lr56/Yr38 linked genes (Marais G F et al., 2010. Euphytica 171:15-22).
It is well accepted that using resistant varieties is the most efficient and economical way to control the leaf rust and stripe rust diseases. However, resistance conferred by many of the currently known genes has been overcome by the pathogenic fungi. In addition, the lines harboring the resistance genes are often inferior in their agricultural traits.
International (PCT) Patent Applications Publication Nos. WO 1995/029238 and WO 1999/045118 disclose genetic sequences which confer or otherwise facilitate disease resistance in plants such as against rust and mildew. The Application provides transgenic plants carrying the subject genetic sequences enabling the generation of disease resistant plants, particularly disease resistant crop varieties.
International (PCT) Patent Applications Publication No. WO 2013/082335 relates to new disease resistant crops and methods of creating new disease resistant crops. Particularly, the Application discloses a wheat genetic line comprising four highly effective disease resistance genes, Lr19, Sr25, Bdv3 and Qfhs.pur-7EL from the wheat-related grasses, Thinopyrum intermedium and Th. Ponticum, all on the long arm of wheat chromosome 7D. The genes are expected to remain in coupling in wheat genetic lines, resulting in wheat genetic lines with reduced susceptibility to yellow dwarf virus, fusarium head blight, stem rust, and leaf rust.
There is a recognized need for and it would be highly advantageous to have commercial agricultural wheat cultivars that are resistant to leaf and stripe rust.